Aneurysms are characterized as a bulging in an artery that results in a thinning of the arterial wall that can lead to rupture. An aneurysm rupture is a potentially life threatening condition. To repair an aneurysm, the surgeon would traditionally remove the diseased arterial tissue and replace it with a cloth replacement tube. This approach is extremely invasive and not an option for some patients.
Endovascular techniques make use of catheters to deliver a stent graft to the diseased site by gaining arterial access through small incisions in the groin or arm. The stent graft bridges the aneurysmal segment of the artery by firmly anchoring in two adjacent healthy segments of arterial tissue. The stent graft is held open by a metal scaffold or “stent” and uses a cloth cover to form a conduit for blood flow that keeps the blood pressure from reaching the diseased tissue. Traditionally, this device has worked well for aneurysms that are in the straight segment of the descending thoracic aorta or in the infrarenal aorta. But stent grafts have been less effective in areas of the branches of the aortic arch, branches of the descending thoracic aorta, or near the iliac branch. In order to repair these branched areas, bridging stents may be utilized. Bridging stents are relatively small diameter stent grafts that span from the main body stent graft to the native branch vessel. These bridging stents have unique requirements from covered stents used for other purposes.
Self-expanding stents can be desirable for complex aneurysm repair for their flexibility. Such self-expanding stents can make bends as they change directions from the main body stent graft to the target branch vessel, and may also exhibit kink resistance. As such, in the event that the aneurysm remodels, the self-expanding stent may avoid kinking and ultimately occluding in addition to allowing some compliance to the remodeling. However, self-expanding stents require an outer sheath or catheter to be pulled back in order to deploy. With no or partial apposition in the branch vessel, the pulling force can pull the stent from the target. This can be problematic, especially if the guidewire is pulled out of the target branch vessel as well. The inherent lack of outward radial force in a self-expanding stent can make this problem worse. One particular type of self-expanding stent that has acceptable radial strength is a woven nitinol wire stent. These woven stents also have exhibit flexibility and kink resistance. However, they suffer from an extreme foreshortening on deployment. This means that they are long and small diameter in the constrained state and short with large diameter in the unconstrained state. Self-expanding stent grafts can be effective with predictable deployments when used to treat stenotic disease in the iliac and superficial femoral artery of in-stent restenosis in dialysis fistulas. But they are not designed for use in aneurysms. In the stenotic case, the walls of the stent graft are in apposition with the arterial wall along the entire length. But in an aneurysm, only the distal extent of the stent graft is in apposition with the vessel wall. So the stenotic case has much more resistance to pull out of the distal end of the stent graft than the aneurysmal case.
Yet another type of stent is a balloon-expandable stent. Balloon expandable stents are disposed on the outside of a balloon of a balloon catheter. When the balloon is inflated, it expands the stent in an outward radial direction. Without a sheath or catheter being pulled back during deployment, the positioning of the stent is predictable and there is less risk of pulling the stent graft out of its intended target vessel.